CHICAGO | A federal investigation of the nation’s largest single-site county jail has uncovered serious sanitation and medical care problems, as well as violence against prisoners who clashed with guards or failed to follow commands, officials said.

Among the problems cited in the 98-page report: Old or mentally ill inmates struck by guards for dressing too slowly, inmates burning milk cartons to heat food in their cells and prisoners rigging a dumbwaiter to move homemade weapons.

Three Cook County Jail inmates committed suicide in the first four months of 2008, and others have died because of inadequate medical care, according to the report, prepared by the civil rights division of the Justice Department and the U.S. attorney’s office after a 17-month investigation.

The Cook County Sheriff’s Office acknowledged in a statement that the investigation had uncovered problems and would serve “as a road map to address operational deficiencies and improve conditions at Cook County Jail for inmates and staff alike.”

But Sheriff Thomas J. Dart also sharply criticized the report, the latest warning that the facility is overcrowded and unsafe. The office said the report “relies on inflammatory language and draws conclusions based on anecdotes and hearsay from inmates.”

“The report’s allegations of systemic violations of civil rights at the jail are categorically rejected by the sheriff’s office,” according to the response. It said that the national jail suicide rate is 24 times that of the Cook County Jail and that the use of force is down sharply.

Mr. Fitzgerald said Thursday that “a culture of abuse” exists, with groups of guards conducting organized beatings of inmates in retaliation for verbal insults. Prisoners were also not protected against violence from other inmates.

Mr. Fitzgerald warned that the federal government could sue to force the county to mend dismal conditions if it delays doing so. So far, he said, jail officials have cooperated.

The report said violence against prisoners sometimes begins as soon as they arrive at the sprawling complex on Chicago’s west side, where nearly 10,000 inmates are housed while awaiting trial.

“Many inmates report that those who are old, mentally ill or do not understand English are struck by officers for undressing or dressing too slowly,” the report said. One prisoner who had trouble complying with orders from guards complained that they used his head as “a bongo drum.”

Overcrowding has resulted in “hot bunking,” in which prisoners use beds in eight-hour shifts. The report said that while each inmate uses his or her own bedding, the practice could still cause “sanitation and infection control problems.”

Skin infections have not been adequately controlled, he said.

Mr. Fitzgerald told reporters that the jail has only one dentist for 9,800 prisoners and that 25 percent of tooth extractions result in infection.

Inmate-on-inmate violence has been a persistent problem, according to the report, including prisoners stabbed, one fatally, with knifelike “shanks,” and another strangled by a cellmate.

“Due to the dilapidated condition of scores of cells, shower areas and various dayroom features, inmates have ample material for fabricating weapons,” the report said.

In one instance, inmates had rigged a dumbwaiter that may have been used to move weapons from tier to tier, it said.

Investigators also said it is common for prisoners to start fires in their cells to warm food, using empty milk cartons and other debris for fuel and light fixtures for ignition.